570 



which has arisen in early Craniate phylogeny in connexion with the 

 degenerating visceral ganglia in front. 



Furthermore it is evident that the mesencephalic flexure ha? had 

 a potent influence upon the evolution of the primitive skeletal frame- 

 work of the head, and on the nature of the extra-myotomic coelomic 

 cavities of the oral and pre-oral region. 



Phylogeny of the Craniate Nervous System. 



The results of the preceding considerations have been brought 

 together in Fig. 7 which is an attempt to indicate the ground plan 

 of the Craniate nervous system. 



The brain as the spinal cord originates from a ciliated invagination 

 of the dorsal ectoderm bearing pigment. An extension of the post- 

 oral collar nervous system of Balanoglossus forwards and backwards 

 would produce the primitive condition which is presented. But it is 

 not suggested that any living form does more than hint at possible 

 modes of origin of the dorsal nerve tube. The only point of im- 

 portance at present is to recognise that the central nervous system 

 arises by an infolding of the dorsal ectoderm producing thereby a tube 

 yielding motor nerves, and associated with a lateral series of sensory 

 ganglia. The invagination always begins anteriorly and extends back- 

 wards with the development of the embryo. The edges of the fold 

 arise and fuse and speedily convert the groove into a ciliated canal, 

 open in front and open also during the period of its formation post- 

 eriorly. The prostomial part of the invagination took place over a 

 region already olfactory and optic in nature the ectoderm of which 

 yielded the elements of the organs concerned and the nervous structures 

 associated with them. In the precraniate state the floor of this region 

 is usually occupied by a sense or cerebral organ. The folding would 

 bring this organ into the centre of the area, and the lateral eyes 

 would be carried up with the folds. The pineal eyes are thus ac- 

 counted for. They are primarily paired structures, but, meeting in 

 the middle line, are usually reduced to one. In front of this region 

 of the primitive head the folding came into close association with the 

 olfactory sensory cells, or involved a region already of an olfactory 

 nature. 



At an early period therefore phylogenetically this region, the 

 original brain, consisted of two segments which arose in connexion 

 with the cerebral ganglia, an anterior olfactory segment opening to the 

 ectoderm and a posterior optic segment bearing above the paired 

 eyes now in close apposition, and below a sensory organ, the infundi- 



